The Biodiversity Crisis
As discussed previously, humans have a notable impact on the Earth. An estimated 83% of the global terrestrial biosphere is under human influence, and perhaps as much as 36% of the bioproductive surface of the Earth is controlled exclusively by man (Harbel and Krausmann 2010). Species diversity represents a dynamic equilibrium between extinction and speciation. Since human colonization, however, this delicate balance has been upset. Evidence from marine ecosystems demonstrates the impact of humans over the past century. During this time 15% of Pacific Island birds have gone extinct, 20 of 297 mussel and clam species and 40 of 950 fishes have perished in North America, amounting to 1 extinction every 20 minutes. The current level of species loss has been compared to that of the late Cretaceous extinction 65mya, in which the dinosaurs and two thirds of species on earth were killed off, possibly due to asteroid impact (Karieva and Marvier 2003).
Anthropogenic activity has a marked influence on trophic skew. By removing species through hunting, fishing down of food webs, elimination of prey, and altering biophysical conditions, dramatic shifts in vegetation composition may occur, causing alterations to trophic levels (Novacek and Cleland 2001).
Another leading cause of biodiversity loss is habitat fragmentation. This is due to both climate change and population expansion and the resulting resource exploitation and alteration of land use patterns. Fragmentation increases local rates of extinction by reducing species population sizes and colonization from similar habitats, eliminating keystone predators or mutualists, enhancing genetic bottlenecks, promoting edge effects, and interrupting landscape-scale processes (Singh 2002). In the future habitat fragmentation is likely to reduce opportunities for speciation and restrict gene flow between species groups. This may be particularly acute amongst larger species such as primates, which are already prone to high rates of speciation and extinction (Levin and Levin 2002).
Severe habitat destruction, overexploitation of populations, freak meteorological events, or the emergence of new disease often results in direct and abrupt species loss, with small populations more likely to go extinct due to these freak events. The final descent into extinction, however, is often driven by synergistic processes that are disconnected from the original cause of species decline (Karieva and Marvier 2003). Habitat degradation and species extinction taking place over short timescales are likely to reset the future evolution of earth’s biota. Evidence from the fossil record suggests that the recovery of global ecosystems takes place over tens of millions of years (Novacek and Cleland 2001).
Ecosystem services
Ecosystem services are benefits to humans from resources and processes that are supplied by natural ecosystems. This definition was formalized in 2004 following the Millennium Ecosystem Assessment. Society is highly dependent on ecosystem products and services for food, shelter and healthcare. As human populations grow, resource demands imposed on ecosystems increase, and the environmental impacts of human ecosystem exploitation; overfishing, deforestation, industrialisation, and landscape degradation become more evident. The relationship between species and the services they provide needs to be understood in order to assess the implications of population change on humanity’s life support systems (Kareiva and Marvier 2003).
Biodiversity is of immense value to human health as ecosystem function and stability are reliant on it. Healthy functioning ecosystems provide humankind with a multitude of economic benefits including timber and fibre whilst being essential for human survival. Constanza et al.(1997) have estimated the value of ecological services to be between $16 and 54 trillion per year. On a global scale, biodiversity represents a balance between rates of speciation and extinction, with greater biodiversity resulting in access to more resources (Singh 2002). If only a few individuals of an endangered species remain, however, they are unlikely to be able to make any meaningful contribution to ecosystem function (Balmford et al. 2003) and will be of considerably lower value.
How to measure ecosystem services
Prediction of extinction risk is dependent upon environmental and biological setting. Diversity is not uniformly distributed on earth, and typically increases from poles to equator (Brook 2008, Singh 2002). Complications arise in estimating the value of ecosystems when unidentified species become extinct. Under such conditions, ecosystem value may be reduced if the loss of these unknown species has a detrimental impact on ecosystem function (Duffy 2003).
Biodiversity hotspots
Tropical ecosystems are home to more unique species than any other habitat, and are hence considered to be biodiversity hotspots - areas containing high concentrations of endemic species. Since the coining of the term ‘hotspots’ by Norman Myers in 1988, research and conservation funding has been focused on these areas, neglecting other species-poor regions such as the Arctic. This is not necessarily an optimal approach, however, as the biological value of ecosystem services should also be considered when deciding which areas are deserving of more attention. Species that are of high value to humans are not found solely in biodiversity hotspot areas, so conservation efforts should perhaps be focused on ensuring that no major ecosystems suffer anything greater than a given percentage of biodiversity loss. It may be equally important to save higher taxonomic groups under threat than areas rich in endemic species, as by only conserving the species in a small area, evolutionary patterns may be altered (Kareiva and Marvier 2003).
Measurement of biodiversity
In measurement of biodiversity, there are 4 key indicators.
· Population richness. The number of populations of a species in a given area.
· Population size. Number of individuals per population, which indicates the frequency distribution of population sizes. It is necessary to understand the contribution of each population to functioning ecosystems.
· Population distribution. The spread of populations relative to their maximum possible extent within an area.
· Genetic differentiation. More genetic variation within populations may provide better resilience to environmental change.
In order to fully understand population change and biodiversity decline, all four indicators must be considered. Biodiversity loss needs to include both species and population-based approaches (Luck et al. 2003).
Human Appropriation of Net Primary Productivity – HANPP, is an indicator that is used to estimate the relative scale of human activities and natural processes. Net primary production – NPP, is the net biomass produced by plants on an annual basis. It can be lost due to human-induced changes in ecosystem productivity, and provides a good indication of trophic energy flows within ecosystems. HANPP represents the extent to which land conversion and biomass harvest change the availability of NPP in ecosystems. Exact definitions of HANPP vary, but Harbel and Krausmann (2010) regard it as being the difference between the amount of NPP available in an ecosystem in the absence of human activities and the amount of NPP remaining in the ecosystem.
HANPP is a significant and useful indicator of human impact for several reasons. It provides a good measure of the physical size of the economy relative to that of the ecosystem. It gives an estimate of what proportion of the potential trophic energy, that could be used for wild animals and other heterotrophs, is still available and indicates human domination of ecosystems. Further to this, utilizing NPP as a basis for ecosystem functioning, human-induced changes of NPP and the way in which they affect patterns, processes and functions of ecosystems can be understood.
Figure 1. Map of Global HANPP. Source: Harbel and Krausmann (2010). http://www.eoearth.org/article/Global_human_appropriation_of_net_primary_production_(HANPP)
Balmford etal. (2003) suggest that the impact of humanity on nature can be gauged using estimations of extinction rates. Habitat loss data is combined with model predictions of changes in species number according to habitat area. Fossil records are incomplete and biased towards abundant and widely distributed species, this limits the applicability of this approach. Further to this, extinctions are difficult to document as only a small fraction of living systems are being fully monitored, there can also be a large time lag between habitat loss and species disappearance and statistical difficulties arise when combining datasets from different studies.
References
Balmford A., Green R.E. and Jenkins M. (2003) ‘Measuring the changing state of nature’, Trends in Ecology and Evolution, 18, 7, 326-330.
Brook B.W., Sodhi N.S. and Bradshaw J.A. (2008) ‘Synergies among extinction drivers under global change’, Trends in Ecology and Evolution, 23, 8, 453-460.
Constanza R. (1997) ‘The value of the world’s ecosystem services and natural capital’, Nature, 387, 6630, 253-260.
Duffy J.E. (2003) ‘Biodiversity loss, trophic skew and ecosystem functioning’, Ecology, 6, 680-687.
Harbel H., Erb K.-H., Krausmann F. (2010) ‘Global human appropriation of net primary production (HANPP), [www], available from http://www.eoearth.org/article/Global_human_appropriation_of_net_primary_production_(HANPP)
Kareiva P. and Marvier M. (2003) ‘Conserving biodiversity Coldspots’, American Scientist, 91, 4, 344-351.
Levin P. and Levin D. (2002) ‘The real biodiversity crisis’, American Scientist, 90, 1, 6.
Luck G.W., Daily G.C. and Ehrlich P.R. (2003) ‘Population diversity and ecosystem services’, Trends in Ecology and Evolution, 18, 7, 331-336.
Novacek M.J. and Cleland E.E. (2001) ‘The current biodiversity extinction event: Scenarios for mitigation and recovery’, Proceedings of the National Academy of Sciences of the United States of America, 98, 10, 5466-5470.
Singh J.S. (2002) ‘The biodiversity crisis: A multifaceted review’, Current Science, 82,6, 638-647.
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